On the role of wind driven ocean dynamics in tropical Atlantic variability

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Abstract

The response of the tropical Atlantic Ocean to wind stress forcing on
seasonal and interannual time scales is examined using an ocean data
assimilation product from the Geophysical Fluid Dynamics Laboratory (GFDL),
and an ocean general circulation model which incorporates a three dimensional
flux correction technique to correct biases of the mean state of the ocean.
On a seasonal time scale, we investigated the impact of the annual
migration of the ITCZ on the exchange pathways of the northern tropical Atlantic.
The results indicate that seasonal variation of the zonal slope of the thermal
ridge along the boundary between the north equatorial countercurrent and north
equatorial current in response to changes in the ITCZ controls, to a large extent,
the amount of water participating in the equatorial circulation. These changes
can be explained in terms of a simple dynamical model where local Ekman
pumping dominates thermocline variation in the western part of the basin, and
Rossby wave adjustment comes into play in the eastern basin.
On an interannual time scale, we examined the upper heat budget of the
equatorial Atlantic in order to identify the key mechanisms by which wind-driven
ocean dynamics control SST variability during the onset and peak phases of the
Atlantic zonal mode. It is found that, in contrast with Pacific ENSO, both
Bjerknes and Ekman feedbacks act together to force the zonal mode, although
their relative importance and dominance depend on season and location.